The basic principles of ion mobility separation (“IMS”) and ion mobility separation devices are well established—generally, ions are urged through a gas-filled region using an electric field and caused to separate by virtue of their differential interactions with molecules of the background or “buffer” gas. The nature and number of these interactions experienced by a particular species of ion will depend on the shape and/or size of the ion and may be described by the ion's effective collision cross section. Known techniques for ion mobility separation include drift tube ion mobility separation, where ions are caused to separate along a constant axial field gradient and travelling wave ion mobility separation, wherein ions are caused to separate by one or more transient DC (or AC) waves.
It is also known to separate or filter ions according to the dependence of their ion mobility on field strength e.g. using an asymmetric periodic DC field. This may be referred to as differential mobility spectrometry (“DMS”) or field assisted ion mobility separation (“FAIMS”).
Ion mobility separation may be used by itself to analyse samples, in which case an ion detector will be disposed at the end of an ion mobility separation device. Alternatively, ion mobility separation may be combined with downstream mass analysis to provide two degrees of separation and e.g. a higher resolution.
It is desired to provide improved methods and devices for use in such ion mobility separation instruments and applications.